One technique that is currently under consideration for the reduction of particulate emissions from diesel engines is exhaust filtration. Various types of filter devices are being studied which could provide high filtration efficiencies with low back pressures.
Some filters also have potential for being cleaned, or regenerated, by incineration of the collected particulates during operation of a vehicle. Depth type filters formed from high temperature materials of various types, particularly fibrous filters, are among those which have been considered for exhaust filtration and have potential for regeneration during operation. Such regeneration may be accomplished by periodically raising the temperature of the engine exhaust gases through means such as engine throttling or auxiliary heating devices.
Our study of the process of thermal regeneration of depth type fibrous filters has shown that three stages are involved in the process: a preignition stage, a rapid combustion stage, and a slow combustion stage.
During the preignition stage, the interaction between the heat generation rate and the heat loss rate produces temperature peaks in the gas and solids which develop and increase in magnitude. When the temperatures are sufficiently high, ignition and rapid combustion being around the temperature peaks.
Shortly after ignition, two combustion fronts are formed which move in opposite directions downstream and upstream. The downstream front propagates rapidly and grows with time. In some cases temperatures at this front may exceed the melting point of fibers in the filter material and cause damage to the filter. The upstream combustion front moves slowly against the direction of gas flow and decays with time.
When the downstream front has burned all the particulate deposit in its path, regeneration continues with only the upstream front operative. This causes a final slow stage of regeneration which decelerates gradually. Thus, regeneration of the inlet section of fibrous depth type filters is relatively difficult and slow.